1. Field of the Invention
The present invention relates to weapons detection, more particularly, to detecting nuclear weapons in cargo containers and vehicles.
2. Description of the Related Art
Perhaps one of the most perilous threats to the United States and the western world is the possibility that a relatively small size nuclear weapon would be transported in a conventional sea cargo container into an American port and detonated there. A rather obvious problem with respect to the inspection of a sea cargo container that might have a nuclear weapon inside is that the weapon could even be detonated while the container ship was in the harbor. Thus the inspection must be done away from the U.S. Probably this will have to be done at the last port of departure before arriving in the U.S.
There are, of course, other methods to transport fully functional nuclear weapon (sometimes referred to as a suitcase bomb) into the US, including but not restricted to, inside airport luggage, cars, trucks, air cargo containers, standard 4′ crates, or indeed being hand carried across the border.
There are several modes of inspection of sea cargo containers in order to detect nuclear weapons materials hidden in a heavily packed a sea cargo container. The apparently simplest method is to detect the gamma rays emitted by the nuclear weapons materials in the bomb, mainly uranium (U) and plutonium (Pu). Plutonium emits a substantial amount of radiation, and uranium emits very little. However as long as 30 years ago, the U.S. Customs Service found that even a substantial quantity of uranium or plutonium in the center of a fully loaded container could not be detected using passive detectors because of the absorption of the emitted gamma radiation by the typical contents.
In addition, there are a wide array of highly-sophisticated methods that use high energy x-rays, gamma rays, or neutrons to impinge on the uranium or plutonium isotopes used in nuclear weapons and then to detect the resulting emissions of neutrons or gamma rays from the excited isotopes of interest. All of these methods have received massive funding over the last few decades. They all suffer from at least two intrinsic and fatal flaws. First, they are, at best, hopelessly expensive in a full scale deployment. Second, they are still ten years away from a successful prototype and the threat is more immediate.
The second mode of inspection using high-energy x-rays is currently widely in operation. This method uses high-energy x-rays, at 3 MeV, 6 MeV, 9 MeV, or even 15 MeV, to image the contents of the sea cargo container. The images are then examined by inspectors to find contraband including nuclear weapons. Some of these systems use two x-ray sources that image from the side and from the top of the container. The evaluation of the two images is quite difficult since it is extremely hard to identify the same object in the two images. Another difficulty is that, even using the highest available peak x-ray sources, the effective penetration of the contents is less than 2′ of iron in a typical sea cargo container that is 8′×12′×40′ long.